#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main()
{
	int money = 0;
	int bottle = 0;
	int water = 0;
	for (money = 20;money > 0;money--)
	{
		water++;
		bottle++;
		if (bottle == 2)
		{
			water++;
			bottle = 1;
		}
	}
	printf("%d\n",water);
	return 0;
}
